Method of fabricating a toothed-shape capacitor node using a thin oxide as a mask

ABSTRACT

The present invention is a method of fabricating a toothed-shape capacitor node in semiconductor DRAM circuit. This invention utilizes dot silicon formed on a nitride layer as an etching mask. The nitride uncovered by the dot silicon is removed. A first layer of poly-oxide is formed using thermal oxidation. The first poly-oxide layer is removed and a second poly-oxide layer is formed using thermal oxidation. The remaining nitride is removed uncovering the polysilicon. The polysilicon is etched to form trenches in the bottom storage of the capacitor. Finally, the second poly-oxide is removed. Thus, a toothed-shape capacitor node is formed in semiconductor circuit.

FIELD OF THE INVENTION

The present invention relates to methods for forming semiconductorintegrated circuits, and more particularly, to a method for formingtoothed-shape capacitor nodes.

BACKGROUND OF THE INVENTION

In recent years there has been a dramatic increase in the packingdensity of DRAMs. Large DRAM devices are normally silicon based, andeach cell typically embodies a single MOS field effect transistor withits source connected to a storage capacitor. This large integration ofDRAMs has been accomplished by a reduction in individual cell size.However, the reduction in cell size results in a decrease in storagecapacitance leading to reliability drawbacks, such as a lower signal tonoise ratio and undesirable signal problems. The desired large scaleintegration in DRAM devices along with reliable operation can beachieved by using DRAM storage capacitors with a high storagecapacitance relative to its cell area.

Efforts to maintain or increase the storage capacitance in memory cellswith greater packing densities have included the use of a stackedcapacitor design in which the capacitor cell uses the space over theMOSFET device area for the capacitor plates. In a recent prior art DRAM,one of the two electrodes of a storage capacitor is formed to have athree-dimensional structure. This makes the capacitance larger by 30% to40% than that of a two-dimensional storage capacitor having the samesize as the three-dimensional one.

For example, a three-dimensional stacked capacitor is disclosed in U.S.Pat. No. 5,053,351. The storage node plate of this capacitor has anE-shaped cross-section. In another example, a hemispherical-grain (HSG)polysilicon storage node has been proposed (see "A New CylindricalCapacitor Using Hemispherical-Grain Si for 256 Mb DRAMs", H. Watanabe etal., Microelectronics Research Laboratories, NEC Corporation). Thismemory cell provides a large storage capacitance by increasing theeffective area of a simple storage node. However, the complex capacitorshapes tend to be difficult to fabricate and the standard processes needat least two mash for the complex capacitor shapes, and moreparticularly, for the toothed-shape capacitor node. Therefore, there isa need for a capacitor node with a high surface area that is simple tomanufacture.

SUMMARY OF THE INVENTION

A method for forming a toothed-shape capacitor node on a semiconductorsubstrate is disclosed. The method comprises the steps of forming apolysilicon layer over said semiconductor substrate; forming adielectric layer on said polysilicon layer; forming dot silicon on saiddielectric layer; removing said dielectric layer uncovered by said dotpolysilicon; oxidizing said dot silicon and said polysilicon layeruncovered by said dielectric layer to form a first poly-oxide layer;removing said poly-oxide layer; oxidizing said polysilicon layeruncovered by said dielectric layer to form a second poly-oxide layer;removing said dielectric layer; removing said polysilicon layeruncovered by said second poly-oxide layer to form trenches in thepolysilicon layer; removing said second poly-oxide layer; and patterningand etching said polysilicon layer with trenches to form a capacitornode.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing aspects and many of the attendant advantages of thisinvention will become more readily appreciated as the same becomesbetter understood by reference to the following detailed description,when taken in conjunction with the accompanying drawings, wherein:

FIGS. 1-8 are cross section views of a semiconductor wafer illustratingvarious stages of forming a toothed-shape capacitor node according toone embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, a single crystal substrate 10 with a <100>crystallographic orientation is provided. In this embodiment adapted forDRAM fabrication, metal-oxide-semiconductor field effect transistors(MOSFETs), word lines and bit lines are formed in and on the substrate10 in any suitable manner well known in the art.

In one embodiment, the word lines and bit lines are formed as follows.Thick field oxide (FOX) regions 12 are formed to provide isolationbetween devices on the substrate 10. The FOX regions 12 is created in aconventional manner. In this embodiment, the FOX regions 12 are formedvia standard photolithography and dry etching steps to define the FOXregions 12 using a silicon nitride layer formed on the substrate. Theexposed portions of the substrate are then subjected to thermaloxidation in an oxygen-steam environment to grow the FOX region 12 to athickness of about 4000-6000 angstroms. The silicon nitride layer isthen removed. Next, a silicon dioxide layer 14 is created on the top ofsurface of the substrate 10 to serve as the gate oxide for subsequentlya formed metal oxide silicon field effect transistors. In thisembodiment, the silicon dioxide layer is formed by using an oxygen-steamambient, at a temperature of about 850°-1000° C. Alternatively, theoxide layer may be formed using any suitable oxide chemical compositionsand procedures. In this embodiment, the silicon dioxide layer is formedto a thickness of approximately 100 angstroms.

A first polysilicon layer is then formed over the FOX regions 12 and thesilicon dioxide layer 14 using a low pressure chemical vapor deposition(LPCVD) process. The first polysilicon layer is doped in order to form aconductive gate for the MOSFET structure. In this embodiment, the firstpolysilicon layer has a thickness of about 500-2000 angstroms and isdoped with phosphorus dopants at a concentration of about 10²⁰ -10²¹ions/cm². A tungsten silicide layer is formed on the first polysiliconlayer to improve interconnection between the gate polysilicon andsubsequently formed metal interconnects. Next, standard photolithographyand etching process are performed to form gate structures 16A and wordlines 16B. Active regions 20A, 20B are formed, using well-knownprocesses to implant appropriate impurities in those regions andactivate the impurities. Sidewall spacers 18 are subsequently formed onthe sidewalls of the first polysilicon layer. Thereafter, active regions24A, 24B (i.e., MOSFET's source and drain) are formed, using well-knownprocesses to implant appropriate impurities in those regions andactivate the impurities. Those skilled in the art of DRAM fabricationcan modify this embodiment to form lightly doped drain (LDD) structures.

Then a first dielectric layer 26 is deposited on the gate structures16A, word line 16B and the substrate 10 for isolation. The firstdielectric layer 26, in the preferred embodiment, is composed of undopedoxide formed Using a standard chemical vapor deposition process to athickness of about 1000-2000 angstroms.

A second dielectric layer 28 is subsequently formed on the firstdielectric layer 26. The second dielectric layer 28 can be formed of anysuitable material such as, for example, BPSG. Preferably, the seconddielectric layer 28 is formed using a conventional chemical vapordeposition process. The thickness of the second dielectric layer 28 isabout 5000 angstroms, but can be any suitable thickness in the range of3000 to 8000 angstroms. The second dielectric layer 28 is thenplanarized to improve the topography for the next processing step. Then,the first dielectric layer 26 and the second dielectric layer 28 ispatterned and etched to form contact windows over the source/drainregions.

Standard processes are then used to form and pattern a photoresist layer(not shown) on the second dielectric layer 28 to define contact holesover selected source/drain regions 24. The photoresist layer leavesuncovered the contact holes, which are then etched through the seconddielectric layer 28, the first dielectric layer 26 to expose a portionof the selected source/drain regions 24. In this embodiment, a standardpatterning and etching process is performed to form the contact hole tohave the minimum width supported by the photolithography process. Theplanarization process performed on the second dielectric layer 28facilitates the formation of the minimum width contact hole.

A second polysilicon layer 32 is then formed on the second dielectriclayer 28 and in the contact hole. The second polysilicon layer 32 isformed using a conventional LPCVD process to completely fill the contactholes. The thickness of the second polysilicon layer 32 on the topsurface of the second dielectric layer 28 is about 3000-8000 angstroms.The second polysilicon layer 32 is doped with phosphorus dopants with aconcentration of about 10²⁰ -10²¹ ions/cm² to increase conductivity. Anysuitable method may be used to doped the polysilicon such as, forexample, in-situ doping. Next, a nitride layer 34 is formed on thesecond polysilicon layer 32. In this embodiment, the nitride layer 34 isformed using any suitable conventional process. The thickness of thenitride layer 34 is about 100 to 400 angstroms. A dot silicon 36 layeris then formed on the nitride layer 34. In this embodiment, the dotsilicon 36 layer can be deposited by any means such as hemisphericalgrained-Si or single Si crystal. The dot silicon 36 is formed usingwell-known processes. For example, the dot silicon 36 can be formedusing a seeding method or traditional HSG deposition process but controlin initial phase reaction. Alternatively, the dot silicon 36 can beformed using conventional chemical vapor deposition method and etchingmethod. In this embodiment, the diameter of the dot silicon 36 is about50 to 500 angstroms and the spacing between the dot silicon is about 100to 1000 angstroms.

Next, the nitride layer 34 left uncovered by the dot silicon 36 isremoved using an etching process. Preferably, the etching process usesan oxide etchant to remove the nitride layer 34. The dot silicon 36serves as an etching mask. The resulting structure is shown in the FIG.2.

Turning to FIG. 3, the dot silicon 36 and the second polysilicon 32 isoxidized to form poly-oxide layer 38A, 38B. The poly-oxide layer 38B isformed on the second polysilicon 32 not covered by the nitride layer 34.The poly-oxide layer 38A is formed on the nitride layer 34 from theoxidation of the dot silicon 36. The nitride layer 34 prevents oxidationof the underlying second polysilicon layer 32. In this embodiment, asuitable oxidation method is a dry oxidation method performed in anoxygen-vapor ambient, at a temperature between about 700° to 900° C. Thethickness of the poly-oxide layer is controlled by the time of theoxidation process. In the preferred embodiment, the thickness of thepoly-oxide layer 38B is about 200-400 angstroms.

FIG. 4 illustrates a next stage of one embodiment of the presentinvention. An etching process is performed to remove the poly-oxidelayer 38A, 38B. In the preferred embodiment, the etching process can beany suitable etching process such as wet etch or dry etch. Afterremoving the poly-oxide layer 38A, 38B, the polysilicon layer 32uncovered by the nitride layer 34 is oxidized once again to form a thinpolyoxide layer 40 on the surface of the polysilicon layer 32. In thisembodiment, a suitable oxidation method is performed in an oxygen-vaporambient, at a temperature between about 700° to 900° C. The thickness ofthe poly-oxide layer is controlled by the time of the oxidation process.In the preferred embodiment, the thickness of the poly-oxide layer 40 isabout 100-300 angstroms. The resulting structure is shown in FIG. 4.

Turning to FIG. 5, the nitride layer 34 is removed. An etching processis performed to remove the nitride layer 34. In this embodiment, theetching process is performed by using a plasma etching process. A wetetching process using a H₃ PO₄ solution may also be performed to etchthe nitride layer 34. Then, the polysilicon layer 32 left uncovered bythe poly-oxide layer 40 is etched back to form trenches in thepolysilicon layer 32. In this embodiment, a standard etching process,such as reactive ion etching is used. The reactive ion etching iscontrolled in time mode. The depth of the trenches are about 2000-6000angstroms. The resulting structure is shown in the FIG. 5.

FIG. 6 shows a cross-section view of the next stage of one embodiment ofthe present invention. The poly-oxide layer 40 is removed using areactive ion etching process. Then, a polysilicon layer 32 with atoothed-shape surface is formed. The resulting structure is shown in theFIG. 6.

FIGS. 7-8 show cross-section views of the final stage of one embodimentof the present invention. The second polysilicon layer 32 with atoothed-shape is masked by a patterned photoresist layer 42 as shown inthe FIG. 7. Then, the second polysilicon layer 32 uncovered by thepatterned photoresist layer 42 is removed. An etching process isperformed to remove the second polysilicon layer 32. Preferably, areactive ion etching process is performed to etch the second polysiliconlayer 32. The second dielectric layer 28 serves as an end-point of theetching process. Then a toothed-shape capacitor node 32A is formed. Theresulting structure is shown in the FIG. 8.

Although specific embodiment has been illustrated and described, it willbe obvious to those skilled in the art that various modifications may bemade without departing from the which is intended to be limited solelyby the appended claims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A method for forming atoothed-shape capacitor node on a semiconductor substrate, said methodcomprising:forming a polysilicon layer over said semiconductorsubstrate; forming a dielectric layer on said polysilicon layer; formingdot silicon on said dielectric layer; removing said dielectric layeruncovered by said dot polysilicon; oxidizing said dot silicon and saidpolysilicon layer uncovered by said dielectric layer to form a firstpoly-oxide layer; removing said first poly-oxide layer; oxidizing saidpolysilicon layer uncovered by said dielectric layer to form a secondpoly-oxide layer; removing said dielectric layer; removing saidpolysilicon layer uncovered by said second poly-oxide layer to formtrenches in the polysilicon layer; removing said second poly-oxidelayer; and patterning and etching said polysilicon layer with trenchesto form a capacitor node.
 2. The method according to claim 1, whereinsaid polysilicon layer is a doped polysilicon layer.
 3. The methodaccording to claim 1, wherein said polysilicon layer has a thickness ina range of about 3000 to 8000 angstroms.
 4. The method according toclaim 1, wherein said dielectric layer is nitride layer.
 5. The methodaccording to claim 1, wherein said dielectric layer has a thickness of arange of about 100 to 400 angstroms.
 6. The method according to claim 1,wherein said dot silicon comprises Hemi-Spherical Grain polysilicon. 7.The method according to claim 1 wherein said dot silicon comprisessingle crystal silicon.
 8. The method according to claim 1 wherein saiddot silicon has a diameter of a range of about 50 to 500 angstrom. 9.The method according to claim 1, wherein said dot silicon has a space ofa range of about 100 to 1000 angstroms.
 10. The method according toclaim 1, wherein removing said dielectric layer is done by using anetching process with an oxide etchant.
 11. The method according to claim1, wherein said first poly-oxide layer has a thickness of a range ofabout 200 to 400 angstroms on said polysilicon layer.
 12. The methodaccording to claim 1, wherein removing said first polyoxide layer isdone by an etching process.
 13. The method according to claim 1, whereinsaid second poly-oxide layer has a thickness of a range of about 100 to300 angstroms on said polysilicon layer.
 14. The method according toclaim 1, wherein removing said second polyoxide layer is done by anetching process.
 15. The method according to claim 1, wherein removingsaid dielectric layer is done by an etching process.
 16. The methodaccording to claim 1, wherein said trenches in said polysilicon layerhave a depth of a range of about 2000 to 6000 angstroms.
 17. A methodfor forming a toothed-shape capacitor node on a semiconductor substrate,said method comprising:forming a polysilicon layer over saidsemiconductor substrate; forming a nitride layer on said polysiliconlayer; forming dot silicon on said nitride layer; etching said nitridelayer uncovered by said dot polysilicon; oxidizing said dot silicon andsaid polysilicon layer uncovered by said nitride layer to form a firstpoly-oxide layer; removing said first poly-oxide layer; oxidizing saidpolysilicon layer uncovered by said nitride layer to form a secondpoly-oxide layer; removing said nitride layer; removing said polysiliconlayer uncovered by said second poly-oxide layer to form trenches in thepolysilicon layer; removing said second poly-oxide layer; and patterningand etching said polysilicon layer with trenches to form a capacitornode.